Toxicity of Pesticides to Pollinators and Beneficials

"Pesticide" is a general term used for a chemical designed to kill target pests such as insects (insecticide), mites (miticide), weeds (herbicide) and organisms which cause plant diseases such as bacteria (bactericide) and fungi (fungicide). Unfortunately, many agricultural pesticides may be toxic to bees. Each year many honey bee colonies are damaged or destroyed by pesticides, primarily insecticides. Such losses have a devastating impact on the beekeeper, who may have to relocate damaged hives or perhaps even be forced out of business. Growers of most insect-pollinated crops (apples, raspberries, cucurbits, alfalfa seed and many others) experience lower yields, and ultimately the consumer must pay higher food prices.

Potential Factors for Honey Bee Damage:

Many factors involving insecticide application affect the potential for honey bee losses. The most important factors are outlined below.

Relative Toxicity of the Chemical: Pesticides vary in their toxicity to honey bees. Most fungicides, herbicides and miticides are relatively nontoxic to honey bees and can generally be used around them without serious harm. The biological insecticide Bacillus thuringiensis exhibits very low toxicity to bees. One group of insecticides which is highly toxic to honey bees cannot be applied to blooming crops when bees are present without causing serious injury to colonies. Among the materials in this high-risk category are diazinon, Imidan, malathion and Sevin.

Choice of Formulation: Different formulations, even of the same pesticide, often vary considerably in their toxicity to bees. Dust formulations are typically more hazardous than sprays because they are picked up on bee hairs. A wettable powder such as Sevin 80S would usually remain toxic in the field for longer time than Sevin XLR Plus, an emulsifiable concentrate. However, granular insecticides are less hazardous to honey bees. Microencapsulated materials such as Penncap-M are particularly dangerous to use around bees because the capsules have a special tendency to adhere to bees due to their size and electrostatic charge, and because the contaminated pollen collected by bees in the treated fields is stored in the hive and remains toxic for an extended period.

Residual Action: Residual activity of an insecticide is an important factor in determining its safety to pollinators. An insecticide which degrades within a few hours can generally be applied with minimum risk when bees are not actively foraging.

Temperature: Temperature can have a substantial effect on the bee poisoning hazard. If temperatures following treatment are unusually low, insecticide residues can remain toxic to bees many times longer than if higher temperatures prevail.

Distance from Treated Fields: The most severely damaged colonies are usually those closest to fields where insecticides are being applied. However, during periods of pollen or nectar shortage, hives within five miles of the treated area can be injured.

Time of Application: Evening application of a short residual insecticide can greatly reduce any potential for bee damage.

Steps to Reducing Damage:

Reducing pesticide injury to honey bees requires communication and cooperation between beekeepers, farmers and applicators. It is important that beekeepers understand cropping practices and pest management practices used by farmers in the vicinity of their apiaries. Likewise, insecticide applicators should be sensitive to locations of apiaries, obtain a basic understanding of honey bee behavior, and learn which materials and application practices are the most hazardous to bees. While it is unlikely that all poisonings can be avoided, a balance must be struck between the effective use of insecticides, the preservation of pollinators and the rights of all--the beekeeper, farmer and applicator. In most cases, bee poisonings can be avoided by observing the following practices.

Do not treat fields in bloom. Be especially careful when spraying crops such as alfalfa, soybeans, and other legumes and pollinating crops. The label of certain insecticides expressly prohibits their application to flowering crops.

Examine fields and field margins before spraying to determine if bees are foraging on flowering weeds such as milkweeds, smartweed or dandelions. Where feasible, eliminate weeds by mowing or tillage.

Choose short residual materials and low-hazard formulations if insecticides absolutely must be applied during the flowering period to save the crop. Notify local beekeepers as far in advance as possible.

Apply insecticides when bees are not foraging. Some insecticides can be applied in late evening or early morning with relative safety. In the case of corn, where bees collect pollen which is shed by tassels in the early morning, short residual materials could be applied from late afternoon until midnight to reduce the bee hazard.

Adjust spray programs in relation to weather conditions. Reconsider the timing of insecticide application if unusually low temperatures are expected that night because residues can remain toxic to bees which enter the field the following day. Cease applications when temperatures rise and bees re-enter the field in early morning. Avoid treating during hot evenings if beehives are very close to the target field and honey bees are clustered on the outside of the hives. Be especially careful that spray does not contact hives.

†Sources: 2010 New England Apple Pest Management Guide and Oregon State University Bulleting PNW591 ‘How to Reduce Bee Poisoning from Pesticides’.
§ Residual Toxicity - The length of time the residues of the product remain toxic to bees after application.
§§ Extended Residual Toxicity - Residues D are expected to cause at least 25 percent EC mortality for longer than 8 hours after F application.a Late evening means after 6-8 PM and assumes that evening temperatures are not unusually high and that bees have stopped foraging. Late evening, night or early mornings means after 6-8 PM, and before 4-7 AM, depending on temperature. Shift time if abnormally high temperatures cause bees to start foraging earlier or continue later than ususal (5:30 AM to 8:00 PM). Few honeybees forage when springtime temperature is below 51˚F. Maximum foraging activity occurs at temperatures above 63˚F. Evening applications are generally less hazardous to bees than early morning applications.
**Where trade names are used, no discrimination is intended and no endorsement by Cooperative Extension is implied. Not a complete list.*Restricted use pesticide; pesticide applicators license required. OMRI certified for organic production